There is a significant demand for the use of inks with exotic and unusual colors, in order to achieve special effects on a variety of writing surfaces. Such special effects often require the incorporation of large and/or dense pigment particles in the writing inks. The use of such pigments, however, creates problems with respect to precipitation of the pigment out of the low viscosity ink carrier, especially after periods of nonuse of the writing instrument.
One of the known devices for applying pigmented inks is the use of squeeze tube cartridge, which is filled with a high viscosity ink. The ink is sufficiently viscous to minimize precipitation of the pigment particles. These instruments are difficult to use, however, and offer little control over the width, and even the precise location, of the intended line. The high viscosity of the ink medium is also disadvantageous in that its penetration into the writing surface is very limited, so that the desired optical effects are not fully achieved. In addition, the resulting heavy lines of high viscosity ink take an excessive amount of time to dry and are subject to smudging. An example of a pen employing a squeezable cartridge tube is the Aoki U.S. Pat. No. 5,688,061, which employs a main body that is flexible, so that the ink can be extruded by squeezing with the fingers. Even so, the pen is provided with a spring-based agitating system to stir the ink and promote dispersion of the pigment.
Conventional ballpoint pens utilize a roller ball in connection with a relatively high viscosity ink which can hold pigment materials in suspension. The ballpoint pen relies upon pressure of the atmosphere to maintain the high viscosity ink in contact with the dispensing ball. When the ball is rotated, a shearing action of the ball surface against the ink body reduces the viscosity of the ink contacting the ball sufficiently to allow the ink to be transferred by the ball from the back of the ball socket to the intended writing surface. The relatively high viscosity of the ink, however, limits its penetration into the paper. Smudging is also a problem with the higher viscosity inks of ballpoint pens. Even with some ballpoint pens, it may be desirable to provide an agitating arrangement to redisperse any coagulated pigment. An example of such is the Nishitani U.S. Pat. No. 6,536,969.
So-called gel pens utilize somewhat lower viscosity ink than the standard ballpoint pens, and thus are an improvement over such ballpoint pens. However, the ink utilized in gel pens is still of relatively high viscosity, so as not to leak from the reservoir or the tip of the pen. The “gel” ink is applied in a manner similar to the ballpoint pen, by the rotating ball subjecting the ink to a shear force to reduce its viscosity as it is being transferred by the ball from the ink supply to the writing surface. While the gel pen is a marginal improvement over the ballpoint pen, it suffers some of the same disadvantages of limited penetration of the ink into the writing surface and some degree of smudging of the applied ink.
Free ink roller ball pens, utilizing low viscosity inks, are in general well known and have been manufactured and sold for many years. Representative such roller ball pens, as made for example by Chartpak, Inc., and also by Pentel, Pilot and Mitsubishi, provide a chamber for low viscosity, liquid ink communicating with a roller ball tip assembly. The ink reservoir has a confined air space above the ink, and the reservoir is maintained in communication with the atmosphere through a pressure equalization chamber, typically a single path or multiple path labyrinthine passageway, such as an injection molded lamella of wetable plastic. This allows air to enter the reservoir as the ink is consumed, and also provides for expansion and contraction of the reservoir air in response to variations in the temperature and/or ambient pressure.
Conventional free ink roller ball pens typically utilize a stainless or tungsten carbide roller balls, manufactured to very tight tolerances and closely received in suitable sockets. Typically, a fibrous feed rod extends up into the ink reservoir, providing a capillary system to maintain the ink supply at the back of the roller socket. It is well known that conventional free ink roller ball pens are unsuitable for use in connection with inks containing pigment that is inherently unstable in the low viscosity ink vehicle. The specialty pigments, desired to achieve effects such as metallic appearance, pearlization, pastel colors, luminescence, thermo-chromic effects and the like, tend to be too large and/or too dense to be retained in suspension in the low viscosity inks over any significant time period, as when the pen is stored between uses.